asset throttling: don't be exhausted if there is no limit

[mockersf]

Objective

• Since Throttle render assets #12622 example compute_shader_game_of_life crashes

thread 'Compute Task Pool (2)' panicked at examples/shader/compute_shader_game_of_life.rs:137:65:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `compute_shader_game_of_life::prepare_bind_group`!
thread '<unnamed>' panicked at examples/shader/compute_shader_game_of_life.rs:254:34:
Requested resource compute_shader_game_of_life::GameOfLifeImageBindGroups does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_event`,
                and can be added by plugins.
Encountered a panic in system `bevy_render::renderer::render_system`!

Solution

• exhausted() now checks that there is a limit